Rail steel

ABSTRACT

A STEEL RAIL SECTION WHICH CONTAINS A GRAIN REFINING ELEMENT AND WHICH HAS BEEN SUBJECTED TO NORMALISING OR CONTROLLED ROLLING TO PRODUCE A FERRITE GRAIN SIZE FINER THAN A.S.T.M. 8 HAS A GREATER RESISTANCE TO BRITTLE FRACTURE THAN CURRENT RAIL SECTIONS ESPECIALLY AT LOW TEMPERATURES WHILE RETAINING GOOD TENSILE PROPERTIES. THE ADDITION OF A HARDENING ELEMENT BENEFITS WEAR RESISTANCE. SUITABLE GRAIN REFINING ELEMENTS ARE ALUMINUM, VANADIUM, TITANIUM, NIOBIUM, ZIRCONIUM, WHILE SUITABLE HARDENING ELEMENTS ARE MANGANESE, SILICON, CHROMIUM, NICKEL AND MOLYBDENUM. CARBON LEVELS CAN BE BETWEEN 0.2 AND 0.85% BY WEIGHT BUT ARE PREFERABLY NOT MORE THAN 0.39%, BELOW THE LEVEL FOR CONVENTIONAL RAIL STEEL. BETWEEN 0.5 AND 2.5% BY WEIGHT MANGANESE OR UP TO 1.5% BY WEGHT SILICON ARE PREFERRED AS THE HARDENING ELEMENTS. FOR GRAIN REFINING IT IS PREFERRED TO USE ALUMINUM OR VANADIUM WITH A NORMALISING TREATMENT, OR ALUMINUM AND NIOBIUM TOGETHER WITH A CONTROLLED ROLLING PROCESS.

United States Patent US. Cl. 148-36 9 Claims ABSTRACT OF THE DISCLOSURE A steel rail section which contains a grain refining element and which has been subjected to normalising or controlled rolling to produce a ferrite grain size finer than A.S.T.M. 8 has a greater resistance to brittle fracture than current rail sections especially at low temperatures while retaining good tensile properties. The addition of a hardening element benefits wear resistance. Suitable grain refining elements are aluminium, vanadium, titanium, niobium, zirconium, while suitable hardening elements are manganese, silicon, chromium, nickel and molybdenum. Carbon levels can be between 0.2 and 0.85% by weight but are preferably not more than 0.39%, below the level for conventional rail steel. Between 0.5 and 2.5% by weight manganese or up to 1.5% by weight silicon are preferred as the hardening elements. For grain refining it is preferred to use aluminium or vanadium with a normalising treatment, or aluminium and niobium together with a controlled rolling process.

This invention relates to steel, and more particularly to rail steel and rail sections fabricated therefrom.

Hitherto, rail steel has chiefly been made with a carbon content of about 0.5% and a manganese content of about 1.0%. The as-rolled rails are normally straightened and machined to length before being put into service. Typical of such rails are those described in the following standard specifications:

British Standard 11:1959 Flat Bottom Railway Rails. Union Internationale des Chemins de Fer U.I.C. 860-0 Technical Specification for the Supply of Vignole (Flat Bottom) Rails in Non-Treated Steel.

, American Railway Engineering Association, Manual of Recommended Practice, Chapter 4, Rail, Part 2, Specification 1968.

In this condition the rails have adequate tensile strength (normally not less than 44 t.s.i., 69 kg./mm. and wear resistance. vHowever, the steel steel structure is generally a coarse grained essentially pearlite structure possessing a very poor impact strength, typically under ft.-lbs. (1.39 kgm.) at room temperature, as measured by the Charpy test using a 2 mm. V-notch specimen. The steel usually has a prior austenite grain size of about A.S.T.M. 3, and a ferrite grain size of about A.S.T.M. 4. Under severe operating conditions, brittle fracture can occur from relatively small defects which may have developed in service e.g. from fatigue cracks which may form from bolt holes through the web of the rail, and also from cracks and defects in the rail.

According to the present invention there is provided a steel rail section which embodies a grain refining element and, preferably, a hardening element and which has been subjected to either normalising or a controlled rolling process, as herein defined, in producing a fine grain structure, the type and amount of the grain refining element in the steel and the fine graining process to which it is subjected being such as to produce a ferrite grain size finer than A.S.T.M. 8, and preferably finer than A.S.T.M. 9.

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As used herein the term normalizing is defined as a heat treatment process in which the steel is reheated to a temperature which is in excess of its upper critical point and is then air cooled. The upper critical point may typically be about 850 C. The term controlled rolling as used herein is defined as a rolling process performed on the steel down to temperatures in the range 700 C. to 900 C., instead of the normal finish rolling temperature of about 1000 C.

The grain refining element may be one or more of aluminium, niobium, vanadium, titanium and zirconium, preferably in the proportion by weight, based on the total weight of the steel, of 0.015% to 0.1% aluminium, 0.01% to 0.1% niobium, or 0.05% to 0.2% vanadium.

Suitable hardening elements are silicon, manganese, chromium, nickel and/or molybdenum. Silicon is desirably present in the proportion, based on the total weight of the steel, of 0.05% to 1.5% by weight; chromium, from 0.25% to 1.5% by weight, and manganese from 0.5% to 2.5% by weight.

Nitrogen is usually present between 0.003 and 0.030%, more preferably with a maximum of 0.025%, by weight, based on the total weight of the steel.

Standard rail steel as currently produced usually has a prior austenite grain size of about A.S.T.M. 3, or a ferrite grain size of about A.S.T.M. 4. By the practice of the present invention using grain refining elements and a normalising or controlled rolling process, a prior austenite grain size finer than A.S.T.M. 6, and a ferrite grain size finer than A.S.T.M. 8, can be produced. The use of grain refining elements combined with the normalising heat treatment process can result in ferrite grain size finer than A.S.T.M. 10, or even A.S.T.M. 12.

By the practice of the present invention it is possible to provide an improved steel which is much more resistant to brittle fracture than current rail steel, down to temperatures of, for example, minus 15 degrees centigrade, whilst still possessing adequate tensile and yield strength and, especially when the hardening elements are used, retaining the wear resistance which is of importance in rail steels.

It has been found that this improved resistance to brittle fracture, which can be measured by improvement in im pact strength, can be provided by the use of the grain refining elements in the steel coupled with either a normalising hea treatment or a controlled rolling process.

A suitable steel rail section in accordance with this invention contains by weight, based on the total weight of the steel:

Carbon: 0.2 to 0.85% Sulphur: 0.06% maximum Phosphorus: 0.06% maximum At least one of the following hardening elements in the stated amounts:

Manganese: 0.5 to 2.5% Silicon: 0 to 1.5% Chromium: 0 to 1.5 Nickel: 0 to 1.0% Molybdenum: 0 to 0.6%

provided that a total hardening element content of 5% is not exceeded; at least one of the following grain refining elements in the stated amounts:

Aluminium: 0.015 to 0.1% Vanadium: 0.05 to 0.2% Niobium: 0.01 to 0.1% Titanium: 0.15 to 0.3% Zirconium: 0.15 to 0.3%

together with nitrogen in an amount between 0.003 and 0.30% and in a substantially stoichiometric proportion to the amount of grain refining elements, the balance being iron and incidental impurities; the steel rail section having been subjected either normalising or a controlled rolling process, as herein defined, in producing a ferrite grain structure finer than A.S.T.M. 8.

In a preferred form the above rail section has a carbon content of from 0.2 to 0.6% and the grain refining element is aluminium, vanadium and/or niobium, the ferrite grain structure being finer than A.S.T.M. 9.

A steel rail section falling within the above definition and being a modification, within the scope of the invention, of steel suitable for forming rail sections in accordance with current rail specifications such as B.S.11: 1959, has the following analysis:

Carbon: 0.4 to 0.6% Manganese: 0.95 to 1.25% Sulphur: 0.06% maximum Phosphorus: 0.06% maximum Nitrogen: 0.003 to 0.030% Silicon: 0.08 to 0.20%

and

Aluminium: 0.015 to 0.1%; or Vanadium: 0.05 to 0.2%; or Niobium: 0.01 to 0.1%

In the absence of vanadium from the above composition the aluminium and niobium contents may be combined to give from 0.015 to 0.10% aluminium together with 0.01 to 0.10% niobium, in which case the rail section is preferably controlled rolled.

These rail sections formed by the addition of grain refining elements to a base alloy similar to that specified in 13.8.11: 1959, and subjected to normalising or controlled rolling, show good impact strengths and resistance to brittle fracture. However, their impact properties are improved in the more preferred embodiments of the invention by using a lower carbon content, specifically from 0.28 to 0.39% by weight of carbon in the alloy.

Such rail sections may contain by weight, based on the total Weight of the steel:

Carbon: 0.28 to 0.39% Manganese: 1.2 to 1.6% Sulphur: 0.06% maximum Phosphorus: 0.06% maximum Nitrogen: 0.003 to 0.030% Silicon: 0.35% maximum and Aluminium: 0.015 to 0.10%; or Vanadium: 0.05 to 0.20%

The most preferred forms of this steel are found in rail sections containing by weight from 0.02 to 0.06% aluminium, or from 0.10 to 0.15% vanadium and from 0.010 to 0.015% nitrogen, the rail section having been subjected to a normalising process; or, alternatively, rail sections containing by weight from 0.015 to 0.10% aluminium and additionally from 0.01 to 0.1% niobium, the rail section having been subjected to a controlled rolling process.

The production of rail sections with a fine-grained structure, as described above, especially when combined with a low pearlite content, will enhance the resistance to brittle fracture. The reduction in pearlite content, and an increase in ferrite content, is achieved by use of the lower carbon contents. This is, however, sometimes accompanied by a decrease in tensile strength and wear resistance. However, in accordance with the most preferred embodiment of the invention, the tensile strength and wear resistance can then be improved by strengthening the increased fraction of the ferrite phase by solid solution hardening by the addition of a hardening element such as silicon, manganese, chromium, nickel or molybdenum, or combinations thereof.

Accordingly, the invention further provides in its most preferred form a steel rail section containing from 0.28 to 0.39% carbon as defined above and containing as a hardening element from 1.2 to 2.5% by weight manganese, and/or from 0.8 to 1.2% by weight silicon. The hardening elements manganese, silicon, chromium, nickel and molybdenum may more generally be included in the ranges previously quoted. In this connection the preferred grain refining elements are aluminium or vanadium, the rail section being normalised, or aluminium and niobium together, the rail section being controlled rolled.

Details of the typical processes performed will now be described by way of example only, in order that the invention may be fully understood.

Steel from a furnace having, for example, the following composition rangecarbon, 0.2 to 0.85%; manganese, 0.5% minimum; phosphorus, 0.06% maximum; sulphur, 0.06% maximum; nitrogen 0.003% to 0.025%, with a balance of iron, apart from the incidental impuritiesis tapped into a ladle into which the grain refining element is added. The temperature of the melt will generally be about 1600 C. The hardening element may also at this stage he added to the melt.

The molten metal may then be teemed from the ladle into an ingot mould, and after solidifying, it may be subjected to one of two rolling processes to form the steel into rail sections:

In the first process, the ingot is heated to about 1300 C. before being fed into a cogging mill. The bloom issuing from this mill is then rolled to a temperature of about 950 C. to 1050" C. In this particular case, the rolled rail may have been produced directly from the ingot without any intermediate reheating; or alternatively the bloom from the cogging mill may have been reheated at temperatures of up to 1300 C. At this juncture, t e rolled rail will be cooled to a temperature below about 700 C. and then normalised, i.e., it is reheated to a temperature of the order of 850 C. to refine the grain and to form precipitates which restrict grain growth, and then cooled in air.

In the second process, the ingot, or the bloom from the cogging mill, is cooled down to at least 700 C., preferably 500 C., and then reheated to a temperature between 1050 C. and 1250 C., i.e. of the order of 1150 C., and the rolling process is continued to give finishing temperatures in the range of 700C. to 900 C. (controlled rolling). The low reheating temperature used in this process results in the presence of grain refining agents during the actual rolling process, and these produce finegrained microstructures of a similar type to those produced during normalising.

Various modifications can be made to the process described. For example, the alloy additions need not be made to the ladle. These additions could be made to the steel making furnace, and, in some instances, additions will be made to the furnace, the ladle and also the ingot mould. Additionally the melt in the ladle may not necessarily be teemed into ingot moulds, and, alternatively, it can be processed through a continuous casting machine or other bloom-forming techniques prior to rolling.

In order that the invention may be more fully understood examples are given in the tables. Table I illustrates chemical composition and grain size, and Table II final mechanical properties.

Example 1 is typical of rail steels (say to 8.5.11: 1959 specification) as currently produced. This example is included for purposes of comparison only, and does not illustrate the present invention. The low charpy impact test results should be noted for comparison with the results achieved in later examples which do embody the present invention.

Examples 2 to 10 relate to steel rail sections in accordance with the invention. In all cases the steels had sulphur and phosphorus contents of below 0.06% and a ferrite grain size finer than A.S.T.-M. 8.

Examples 2, 3 and 4 are typical of the results achieved by making additions of grain refiners to the normal quality (say B.S.11: 19'59) rail steel, and subsequently normalising the rolled rail section. Examples 2A and 2B show the use of aluminium, Example 3 vanadium, and Example 4 niobium.

Examples 5, 6, 7 and 8 demonstrate the good impact properties which are achieved by the use of a lower carbon, higher manganese content, together with the addition of grain refining elements and subsequent normalising of the rolled rail section. Examples 5 and 6 illustrate the use of aluminium as grain refining element, and Examples 7 and 8 illustrate the use of vanadium. Examples 6 and 8 also show the inclusion of higher silicon contents in the steel as a means of achieving improved wear resistance.

Examples 9 and 10 illustrate the use of aluminium together with niobium as the grain refining elements, the rail sections being controlled rolled. In Example 9 the composition corresponds to conventional rail steel with the addition of the grain refining elements, while in Example 10 the composition corresponds to the preferred lower carbon, higher manganese steel.

TABLE I together with at least one of the following hardening elements up to the stated amounts:

Manganese: above 0.5 up to 2.5% maximum Silicon: 1.5% maximum Chromium: 1.5% maximum Nickel: 1.0% maximum Molybdenum: 0.6% maximum provided that a total hardening element content of 5% is not exceeded; and at least one of the following grain refining elements in the stated amounts:

Aluminium: 0.015 to 0.1% Vanadium: 0.05 to 0.2% Niobium: 0.01 to 0.1% Titanium: 0.015 to 0.3% Zirconium: 0.015 to 0.3%

together with nitrogen in an amount between 0.003 and 0.030% in a substantially stoichiometric proportion to the amount of grain refining elements, the balance being iron and incidental impurities; the steel rail section having an essentially ferritic/pearlitic structure with a ferrite grain size finer than A.S.T.M. 8.

2. A steel rail section according to claim 1 consisting essentially of the following elements in the stated proportions by weight, based on the total weight of the steel:

Carbon: 0.4% to 0.6% Manganese: 0.95 to 1.25% Sulphur: 0.06% maximum Phosphorus: 0.06% maximum 1.15 1.20 Nitrogen: 0.003 to 0.030%

Silicon: 0.08 to 0.20% {13 and one grain refining element selected from 0.015 to H3 8 0.1% aluminum, 0.05 to 0.2% vanadium and 0.01 to 0.1% niobium; the balance being iron and incidental 1. 2% g impurities. 10 3. A steel rail section according to claim 1 consisting i-g; 3 40 essentially of the following elements in the stated propor- 1 9 tions by weight, based on the total weight of the steel: 1.15 7 1.47 7 Carbon: 0.4 to 0.6%

Manganese: 0.95 to 1.25%

TABLE II Tensile properties Charpy impact test- Lower yield Ultimate' 'tensile At 15 C. 20 ft. lbs. stress stress Elon- (2.76 kg.m.) gation Reducisothermal to tion of transition break area tem era- Example No T.s.l. kgJmm. T.s.i. KgJmm. percent percent Ft. lbs. Kg. m. ture( C.)

30. 1 47. 4 45. 5 71. 7 22. 2 62. 4 22 3. 0 -20 26.7 42.1 45.9 72.3 23.3 43.2 2 0.3 30. 0 47. 2 45. 2 71. 2 32. 0 62. 8 28 3. 9 36 30. 0 47. 2 45. 0 70. 9 32. 0 62. 0 34 4. 7 -42 30. o 47. 2 46. 0 72. 4 2s. 0 57. 0 29 4. 0 32. 0 50. 4 50. s 80. o 26. 0 51. 2 16 2. 2 +6 Carbon: 0.2 to 0.85% Sulphur: 0.06% maximum Phosphorus: 0.06% maximum Manganese: 0 to 0.5%

Sulphur: 0.06 maximum Phosphorus: 0.06% maximum Nitrogen: 0.003 to 0.030% Silicon: 0.08 to 0.20% Aluminium: 0.015 to 0.10% Niobium: 0.01 to 0.10%

the balance being iron and incidental impurities; the steel having been subjected to a controlled rolling process.

4. A steel rail section according to claim 1 wherein carbon is present in an amount from 0.28 to 0.39% by weight, based on the total weight of the steel.

5. A steel rail section according to claim 4 wherein the hardening elements are from 1.2 to 1.6% by weight manganese and up to 0.35% by weight silicon, and the grain refining element is one selected from 0.015 to 0.10% by weight aluminium and 0.05 to 0.20% by weight vanadium.

6. A steel rail section according to claim 4 wherein the hardening elements are from 1.2 to 1.6% by weight manganese and up to 0.35 by weight silicon, and the grain refining elements are from 0.015 to 0.10% by weight aluminium together with from 0.01 to 0.1% by weight niobium, the rail section having been subjected to a controlled rolling process.

7. A steel rail section according to claim 4 wherein the hardening element is selected from 1.2 to 2.5% by weight manganese and from 0.8 to 1.2% by weight silicon.

8. A steel rail section according to claim 7 wherein the grain refining element is selected from aluminium and vanadium, and the rail section has been normalised.

9. A steel rail section according to claim 7 wherein the grain refining elements are aluminium together with niobium, the rail section having been controlled rolled.

References Cited UNITED STATES PATENTS 2,793,947 5/1957 Swanson 75-126 3,519,497 7/1970 Pomey 148-134X 3,207,637 9/1965 Matuschka 75-124X 2,576,223 11/1951 Hofmann 148-12.4 3,562,028 2/ 1971 Heitmann et a1. 75-423 B 2,012,765 8/1935 Marthourey 14836 FOREIGN PATENTS 1,174,515 7/1964 Germany 75-123 M CHARLES N. LOVELL, Primary Examiner US. Cl. X.R.

75123 R. 124. 126 R. 128 R 

